This application claims the priority of German Application No. 198 06 516.7, filed Feb. 17, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for sterilizing containers in an evacuable reactor by way of a low pressure plasma, as well as to a device for carrying out the process.
U.S. Pat. No. 4,207,286 discloses that, in the reactor housing the containers, a low pressure is created and an alternating current plasma is sourced. The frequency sourcing can take place either in a capacitive or inductive manner via a high frequency generator or alternatively by way of a microwave generator. The containers in the reactor are already in a state which permits sterilization by the low pressure plasma.
Such prerequisites are, however, not on hand in certain areas of application. In the pharmaceutical industry or in filling re-usable bottles, for example, it is necessary to clean the usually glass or plastic containers before sterilization by washing them. Before they reach the sterilizing installation, the containers go through a washing machine, after which they are still covered with small amounts of rest water. In this respect it should be noted here that in standard thermal or wet-chemical aseptics, the rest water presents no problem, as the containers are again wetted with process fluids, or in the case of thermal sterilization, where the rest water evaporates anyway during the process.
Rest water, however, as well as other layers found on the container surfaces, hinder the process of sterilization by a low pressure plasma. The plasma then cannot reach the surfaces to be sterilized nor the germs contained in the fluids.
When the reactor is evacuated to the pressure level required for sterilization, the boiling point of the water drops to such a degree as the pressure is reduced such that the rest water, starting at the surface, subsequently starts to evaporate already at room temperature. The energy required for this is taken predominantly from the lower lying fluid layers, which can then freeze over. The resulting layer then present on the container surfaces renders plasma sterilization impossible.
An object of the present invention is to use a low pressure plasma in a reactor to sterilize containers which have been previously washed and which subsequently have rest water amounts on their surfaces.
This object has been achieved in accordance with the present invention in that the containers in the reactor are first of all dried by microwaves and that the plasma is ignited only after the drying process is completed. The drying process can occur at atmospheric pressure, but also completely in a vacuum.
In contrast to the above mentioned prior art, the microwaves in the case of the present invention do not serve the actual process of sterilization, but rather the preparation of the containers for the sterilizing process, namely the removal of rest water amounts. Thereby, the surfaces of the containers are put into a state whereby they can be subsequently sterilized by a low pressure plasma at a low temperature. Drying off of the rest water amounts by microwaves can be carried out speedily without the containers heating up greatly, so that a cooling down in preparation for a possible filling process is not necessary.
The ice layer formed from rest water during evacuation of the reactor absorbs the microwave energy and becomes warm. The pressure in the reactor, and thus the boiling point of the liquid, can be selected at such a low level that, at the point of liquification, the ice may already evaporate, that is, sublime. As in particular in a sublimation process, no liquid and thus no heatable water is present on the surface of the containers, the containers cannot therefore be heated up by the microwaves. In addition, the evaporating liquid continuously withdraws the thermal energy from the lower lying ice layers and thus cools the same. After all water and ice has been removed, almost no more microwave energy is absorbed in the reactor, so that the microwave energy density increases, as at first, the microwaves are still beamed in. The pressure in the reactor also rapidly drops significantly due to the absence of water vapor, insofar as during the process, the suction action remains constant.
Both these aspects, namely dropping pressure and increasing energy density, can be utilized for controlling the ignition of the plasma necessary for actual sterilization. It would appear most purposeful to determine the end of the drying process by monitoring the pressure, that is by measuring the absolute values of the pressure and/or by measuring the time dependance of the pressure course. Here care should be taken that during or after the end of the drying process, it does not come to a premature, undesired ignition of a microwave plasma due to the increasing energy density. The microwave plasma may locally ignite and thus may result in local overheating and possibly in damage to the containers or the reactor components.
In order to generate and maintain the plasma required for sterilization, a high frequency generator is switched on and the microwave generator is switched off at the latest after the plasma has been ignited. High frequency generators, whose frequency can be either capacitively or inductively sourced, function, for example, at a permitted frequency of 13.56 MHz. Containers can be sterilized using an alternating current plasma generated in this way. The microwave generator, not needed while the plasma is maintained, and which may be switched off before the plasma is ignited, operates in contrast at a higher frequency, for example, at a permitted frequency of 2.45 Ghz.
For supporting plasma ignition, the present invention switches off the microwave generator only after the high frequency generator has been switched on. This is particularly helpful in the case of relatively large and high reactors, when the high frequency generator cannot generate sufficient field strength for the ignition. In this case, a momentary, local break-through generated by microwaves can significantly facilitate the ignition of the sterilization plasma. A high frequency generator can be more economically designed when it does not have to supply the ignition field strength, which is significantly higher than that required to maintain a discharge. Large reactors according to the present invention occur than, for example, when the evacuable space amounts to more than 0.1 m3 or when the height of the reactor is higher than around 10 cm.
In order to source the necessary microwave power in the reactor during the drying process, the impedance of the charged reactor must be adapted to match the characteristic wave impedance of the microwave generator and the waveguide. For this purpose, an impedance converter, for example a triple screw transformer, is provided, which is applied in close proximity to the reactor. As the reactor impedance can alter very quickly due to the decreasing amount of water or ice during the drying process, a temporary mismatch cannot be avoided. This effects the reflection of a more or less large part of the power delivered by the microwave generator back to the microwave generator which usually takes the form of a magnetron. In order to protect the microwave generator, a so-called circulator or the like which is mounted between the impedance converter and the microwave generator and deflects the reflected wave into, for example, a water resistor.
In a further embodiment of the present invention, a vacuum buffer is provided between the reactor and the vacuum pump in order to accelerate the evacuation of the reactor.